Aerospatiale AS350 B2 ECUREUIL accident near Yukon, Oklahoma, May 12, 2021

The NTSB's narrative final · quoted from the NTSB record

The pilot receiving instruction (pilot) and the flight instructor were conducting a training flight in the helicopter. They performed several simulated emergencies, each of which required the helicopter’s hydraulic system to be turned off and then turned back on at the conclusion of the procedure. The hydraulic system was turned off and on using the hydraulic cut-off switch, an unguarded push-button switch mounted on the end of the pilot’s collective stick. After completing the emergency procedures, the pilot performed four quick stop maneuvers. The flight instructor reported that on the last quick stop, the helicopter slowed normally but then started a left yaw about 25 ft above ground level. The pilot noted the left yaw and attempted to correct it, but his pedal inputs did not stop the yaw. As the pilot tightened his grip on the collective, the hydraulic system turned off, likely due to the pilot inadvertently pressing the hydraulic cut-off switch, and the left yaw rapidly increased to a left spin. According to the flight instructor, the control loads “instantly became excessive,” and he noticed the hydraulic light on the caution warning panel was illuminated. The pilot intentionally pressed the hydraulic cut-off switch a total of three times, but hydraulic pressure was never restored. The flight instructor told the pilot that he was taking control of the helicopter. However, the pilot did not relinquish control. The flight instructor attempted to regain control of the helicopter but was unable to overcome the high control loads. The helicopter continued to spin, impacted the ground, rolled over, and came to rest on its right side. A postimpact fire consumed most of the helicopter. Although examination of the helicopter was limited due to fire damage, no preimpact abnormalities were identified with helicopter’s airframe and engine. The US Customs and Border Protection Air and Marine Operations Division reported that the agency’s selection process for the Air Interdiction Agent Program failed to properly identify that the pilot was not qualified for the program. Because the pilot did not have the qualifications and experience required for the Air Interdiction Agent Program, he did not have the prerequisite skill necessary to critically assess the situation given by the flight instructor. This resulted in the pilot applying improper corrective actions and failing to relinquish control of the helicopter to the flight instructor when directed, which contributed the accident.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.